1. Field of the Invention
The present invention generally relates to a method for adhering a semiconductor chip to a supporting substrate and, more particularly, to a solder ball limiting metallurgy (BLM) process for improving the integrity of solder joint interconnections between a substrate and a chip.
2. Description of the Prior Art
Solder ball connections have been proven very successful for electrically connecting a semiconductor chip to a supporting substrate. Since the substrate surface is solder non-wettable, Pb/Sn (lead/tin) solder does not bond to the substrate surface directly. Hence, an intermediate pad structure must normally be used between the two surfaces to facilitate adhesion. U.S. Pat. No. 5,027,188 to Owada et al. shows an example of this type of solder joint wherein an adhesion pad, commonly known as ball limiting metallurgy (BLM), is interposed between a chip and, for example, an Sn/Pb alloy solder ball.
The solder ball fabrication could be done by use of either a dry or a wet process. In the conventional dry process, as described in U.S. Pat. No. 4,434,434 issued to Bhattacharya et al., the BLM and the solder are sequentially evaporated on the wafer surface through a thick metal or a photoresist mask. The BLM metalization typically consists of three layers of metalizations deposited in a single pump down cycle. These composite layers consist of an adhesion layer at the bottom and a solder wettable layer at the top, with an intermediate phase layer (a mixture of adhesion and wettable metalizations) between the two layers. The Cr/Phased Cr--Cu/Cu/Au metalization is often used as the BLM material and Pb-low Sn solder is a commonly used solder (sometimes called C-4 solder). Since the solder interconnection lies on top of the thin insulator layer on the chip surface and the joint is often subjected to mechanical and thermal stressing, the damage to the insulator could occur due to a localization of film stress within the joint. Specifically, the insulator damage could occur quite readily if the edges of the Cr and the Cu layers coincide. For this reason, an edge separation is needed to be maintained by extending the Cr edges beyond the Cu edges. This is done by a proper placement of the charges within the evaporator. This edge separation is needed for both the dry and the wet (plating) processes to maintain the mechanical integrity of the joint.
U.S. Pat. No. 4,360,142 to Carpenter et al. discloses a modified dry process, which uses a six layer BLM: CR/Cr-Cu phased/Cu/Cr--Cu phased/Cu/Au. During solder melting Au dissolves readily in the solder and the top Cu layer is wetted by the solder. However, due to the presence of an intervening top Cr--Cu phased layer, the bottom Cu remains unreacted. Carpenter et al. teaches that the thermal gradient in the joint can be reduced by the presence of the unreacted Cu, an excellent heat conduction material. The resulting reduction of the thermal gradient will greatly reduce the susceptibility to thermomigration, a process of atom depletion induced by the presence of a thermal gradient.
U.S. Pat. No. 4,360,142 discloses an alternative method of BLM fabrication. In this process, a blanket deposition of six layers of film is done on the wafer. Following deposition of the last layer, a layer of photoresist is formed on the surface, exposed to the desired pattern and developed. Suitable etching solutions are used to remove the exposed areas of all six layers of film to define the BLM. Subsequently, the solder is deposited via a metal mask over the BLM followed by the reflowing of the solder ball over the BLM pad. This process does not provide Cr--Cu edge separation and thus the insulator damage could readily occur in the disclosed structure.
In the wet process the solder deposition is made by electroplating the Pb-Sn solder on top of the blanket BLM metalization through the holes in a thick resist. At first, a blanket layer of Cr/phased Cr--Cu film is deposited on top of the wafer surface. This blanket film layer serves as the electrical contact for the electroplating. A thick layer of resist is laminated over the blanket BLM film and then hole patterns are generated by conventional lithography processes. By using the blanket film layer as a common electrical contact, the solder is then electroplated at the holes on top of the metalization layers. Next, the photoresist is stripped and the excess metalization layers are then chemically etched using the solder deposit as a mask. The etching is done sequentially by first removing the Cu layer, followed by the phased layer and then the Cr layer. Finally, in order to provide the edge separation, the Cu is etched off selectively. FIG. 1 shows this process results in an overhang of the phased layer. Specifically, FIG. 1 shows a solder wettable layer 16, intermediate phased layer 14 and a solder non-wettable layer 12 deposited on a supporting substrate 10. The wettable layer 16 and the non-wettable adhesion layer 12 are composed of different metals with the intermediate layer 14 being a phased combination of the two. A solder mass 18 is deposited over the structure. The actual BLM pad structure is defined after a series of electrochemical and chemical sub-etched processes using the solder mass 18 as a mask. The resulting structure consists of the phased layer 14 extending beyond both the non-wettable layer 12 and the wettable layer 16. As explained above, when heated, the solder mass 18 reacts with and adheres to the wettable layer 16. The layer arrangement of FIG. 1 is particularly undesirable since molten solder will come into contact with the extending edges of the phased layer 14. The phased layer 14 is a composition of wettable and non-wettable metals and could be wetted by the solder. Thus, the structure does not provide adequate edge separation between the wetted phased layer and the Cr layer. As a result, high stress concentration would develop at the BLM edges, causing insulator damage.